If you're adding audio, you're probably adding a file (which has to be changed to 48kHz) or audio from a video clip. If you want to do a voiceover, you can record it on the video camera and capture that clip the same way you do a regular video clip. In fact, you'll treat it the same, and keep the audio while deleting the video (right-click on the clip and choose Unlink Audio and Video, then delete the video).
Say you want to overlay narration on background audio, you'll want to lower the volume of the background audio. To work with the audio levels, from the menu, choose Window > Audio Mixer. Each channel strip corresponds to an audio track in the Timeline.
To change the volume, drag the Volume slider in the channel strip to increase or decrease the audio level.
You can adjust audio levels in the Audio Mixer while the Timeline is playing! This is often the best way to do things: lay down all your audio and then play it back through the Audio Mixer.
To make changes during that playback, turn on Automation Write (pencil icon) at the top of each Audio Mixer track. Then use the play button in the Audio Mixer window and as the audio plays, use the sliders to change the audio levels. To listen to changes you've made, change the tracks to Automation Read mode (glasses), move the Edit Line back before the audio changes, and hit play. You'll hear your changes, and see the sliders move.